The present invention relates to a data communication system including a packet switching network, a plurality of user circuits, and a plurality of sender/receiver circuits each coupled between said network and at least one of said user circuits, each sender circuit of an originating sender/receiver circuit being able to transmit packets received from an originating user circuit on said network and each receiver circuit on a destination sender/receiver circuit coupled to a destination user circuit including a timing circuit to subject received packets to a delay.
Such a system is already known from the article "Analysis and Optimal Design of a Packet-Voice Receiver" by G. Barberis and D. Pazzaglia, published in the IEEE Transactions on Communications, Vol. Com. 28, No. 2, February 1980, pp 217-227.
In this known data communication system the sender generates data packets from talk spurts produced in an originating user circuit. As these talk spurts are separated by silent periods a set of distinct series of data packets is produced. In a first embodiment of this known system no information is gathered on the delay to which the packets are subjected in the network and a clock in the receiver circuit is set in accordance with the clock in the sender circuit, a time interval equal to the above delay after the receipt of each first packet of a series. As the network delay is a random value it might happen in a worst case, i.e. when the network delay suffered by a first packet of a series is very small, that one or more following packets of the series do not arrive in time at the receiver because they have suffered a too large delay in the network. For each of these packets one waits until it is completely received before feeding it to the destination user circuit. Thus each of these packets is subjected to an additional variable delay which adversely affects the operation quality of the system. When, in a worst case, the maximum of the network delays become larger than the time interval separating the series under consideration from the following one in the set, it is clear that bits will be lost. In the above mentioned article such a worst case is not considered, perhaps because a reduced quality is allowable in the case of voice and maybe especially because the occurrence of such a worst case is then very doubtful as the average time interval between two consecutive series of packets is equal to the duration of each of these series, as mentioned in the above article. Also, it only affects the quality of the relatively small corresponding series of packets and not of the other ones.
However, this would not be the case if the series of packets were relatively very long and if the nature of the data was unknown, i.e. constituted by a continuous bitstream of voice, video or other data or a mixture thereof. Indeed, then the loss of bits would not be allowable since these bits could have a high importance and because the reduced quality would then affect the whole series of packets.